Many subsurface resources, such as oil bearing formations, can no longer be exploited by drilling wells having vertical boreholes from the surface. Extended reach wells, such as wells drilled from platforms or "islands" and having long non-vertical or deviated portions, are now common. A deviated hole portion is defined as one having an axis in a direction at a significant incline angle to the vertical or gravity direction. The inclined portion is typically located below an initial (top) portion which is nearly vertical. A deviated (lower) portion may have an inclined angle from the vertical that may approach 90 degrees (i.e., nearly horizontal). The result is a well bottom laterally offset from the top by a significant or extended distance.
Conventional drill bit technology can produce a borehole portion at almost any incline angle, but extended reach operations at inclined angles have experienced problems. Drilling problems are primarily caused by drag forces on the long and non-vertical drill string rotating the drill bit. More severe problems can occur when a long casing or liner strings are slid and set in highly deviated (pre-drilled) wells.
Casing or liner strings are generally larger and heavier than a rotating drill string. In an inclined well bore, the larger diameter casing and liners tend to increase contact and bearing weight, dramatically increasing drag forces. Because of this increased drag, the torsional forces that would be needed to rotate the casing or liner can be greater than the torsional strength of the pipe itself, or greater than the available rotary torque. Casing or liner strings are therefore normally run (i.e., slid) into the hole without drag reducing rotation.
This non-rotational drag problem is the result (at least in part) of the differences between the static and sliding coefficients of friction. A static coefficient of friction is almost always significantly higher than a sliding coefficient of friction. Thus, rotation during drilling creates constant (sliding) motion, preventing higher static friction even if the drill string is not moving in an axial direction. But once rotation and axial motion within the well bore stops or is interrupted, as in the cases of running casing or orienting a drill string, significantly higher forces may be required to move tubulars because of the difference between sliding and static coefficients of friction.
Drilling or completing an extended reach (or long deviated portion) borehole can exacerbate normal drag forces and problems in addition to inclined well drag forces. Interruptions in drilling or running liner/casing are more likely in an extended reach well. Risk of sticking is especially high if the incline angle exceeds a critical angle. A critical angle produces more drag force than the component of weight tending to slide the casing or liner down the hole. Even if a stuck string is avoided, the forced needed to overcome high drag may cause serious damage to the pipe. These problems create high risk for extended reach wells.
Although high risk, producing some resources through long horizontal well portions or intervals can be very profitable. Fluid production from low permeability and/or thin bed reservoirs may not be economic using near vertical wells but profitable using an extended reach well. Production from shallow resources in fields having limited surface access may not even be possible without extended reach boreholes. For example, an offshore drilling site near an offshore resource may be unlicensable. The ability to reach out to an offshore resource from an on-shore site may mean the difference between an unavailable resource and a producing reservior.
Even for fields where reservoir access or low permeability is not a problem, long, nearly horizontal well portions may be economically desirable in spite of high cost and risk. Significantly production may be possible from extended horizontal well portions in a vertically permeable layer where vertical wells produce excessive unwanted water from adjacent layers, i.e., a large coning effect occurs.
Options to mitigate high drag related problems in extended reach wells are available. For example, high drag mitigation methods can: 1) add shocks and forces to tubulars near the surface, e.g., using a bumper subassembly or adding surface weights; 2) reduce the coefficient of friction, e.g., by lubrication or conditioning of the borehole; or 3) reduce drag-inducing (normal-to-the-borehole) wall forces.
However, these mitigation options are generally costly and may not be effective for many extended reach wells. For example, only a bounded amount of added weight can be exerted at the top vertical portion of a long pipe string before structural limits are reached. Excessive downward force which would be needed for extended reach wells also tends to buckle the string, adding still further drag forces (if laterally supported in a highly deviated well bore) prior to causing structural failure. Adding weight in the inclined portions beyond the critical angle only increases drag. In addition, drilling with large added downward forces may be impractical or rig pick up weight limits may be exceeded.
Similar limits affect methods for reducing the coefficient of friction (e.g., hole conditioning, drag reducing, or lubricating). As longer pipe strings are run into an extended reach well, even a lubricated string will eventually generate unacceptable drag forces because friction is only reduced, not eliminated. The geometry and borehole wall conditions (e.g., interface surface) of some holes may also create increased sliding resistance (high drag) conditions even with lubricated strings.
A flotation method of placing a pipe string into a deviated, liquid-filled hole is also known which lowers drag by reducing forces normal to the wall. This buoyant or floatation method is illustrated in U.S. Pat. No. 4,986,361. After providing a means to plug a pipe string portion, the portion is filled with a low density fluid to provide a buoyant force, reducing the forces acting normal to the walls in nearly horizontal portions. However, the low density fluid normally must be removed from the pipe string after feeding the plugged string into the well bore and prior to cementing. Cementing operations are then typically accomplished without a low density fluid providing a buoyant force.
It would be desirable to further reduce the drag forces associated with moving an entire drill string or casing string. The drag forces require large weights or force application apparatus and high string strengths.